Electro-convective diffractive device

ABSTRACT

A method for producing a diffraction grating is provided. First, a mixture including nematic liquid crystal, dopant, and polymerizable precursor is introduced between two electrically conductive substrates having alignment layers for inducing orientation of the liquid crystal director. A potential difference is applied across the liquid crystal to cause a spontaneous self-assembly of the liquid crystal into an array of convective rolls. Thereafter, the roll structure is stabilized by the creation of a polymeric network through polymerization and/or cross-linking of the polymerizable precursor. The convective roll structure serves as a template for the formation of the polymeric network.

BACKGROUND OF THE INVENTION

The present invention generally relates to diffraction gratings,particularly electro-convective diffractive devices. The presentinvention also relates to a method for fabricating such diffractiongratings through spontaneous self-assembly of liquid crystals intoarrays of convective rolls.

Adaptive electro-optic devices are of paramount importance forinformation collection from the environment. In military applications,this involves both detection and evasion of weapons platforms. Becausedetection technologies now exploit ever-larger regions of theelectromagnetic spectrum, evasion technologies must keep pace, andwavelength-agile devices that operate in the sub-millimeter infraredband are of burgeoning importance. Furthermore, there are many civilianapplications for advanced diffraction gratings. Such applicationsinclude multi-spectral imaging for environmental remote-sensing,wavelength division multiplexing for communications, and spectroscopy.

Diffraction gratings are integral parts of an enormous variety ofdevices that employ electromagnetic radiation. A diffraction grating isrealized when a material possesses a spatially periodic modulation ofeither the real or imaginary parts of the refractive index. When anelectromagnetic wave encounters this material, the spatial modulationcouples with the wavelength of the electromagnetic wave to deflect thewave in a manner depending upon its wavelength. Thus, the spatialmodulation and, more specifically, the periodicity of the modulation, aswell as the wavelength of the electromagnetic wave, determine the pathof the electromagnetic wave through this material. This property makesgratings useful for separating multi-wavelength electromagnetic wavesinto their constituent wavelengths. Additionally, these diffractionsgratings are useful for steering light beams by deflecting them.

The most common method for making diffraction gratings is the replicatechnique, in which a regular pattern of lines is scribed on a moldingsurface. This molding surface is then applied to a blank, usually ofplastic, and the pattern is replicated on the plastic surface. Gratingslike this are inexpensive and durable, but cannot be changed in anyfashion after they are made. Other technologies employing chiral liquidcrystals have also been used. A notable example is U.S. Pat. No.6,188,462, wherein the spatial periodicity needed to form a gratingderives from the intrinsic molecular properties of cholesteric liquidcrystals. The spatial periodicity can be adjusted somewhat by applyingan electric field, but the capability for more substantial adjustment indiffraction gratings is still desired in the art.

The present invention provides a dynamic method for spontaneousself-assembly of diffraction gratings. The grating spacing (also calledthe grating constant) and structure factor of the diffraction gratingscan be tuned by the amplitude and frequency of an applied electricfield.

SUMMARY OF THE INVENTION

When subjected to an externally applied electric field, nematic liquidcrystal maintained between substrates can spontaneously self-assemblyinto a regular array of convective rolls—a phenomenon known aselectro-hydrodynamic convection. This is generally depicted in FIG. 1,wherein a electric field is applied across substrates with transparentelectrodes 1 and 2, which are spaced from each other by a distance d.Convective rolls of liquid crystal 3 are formed between transparentelectrodes. 1 and 2. In a “convective roll” (such as convective roll 3),the liquid crystal flows in a rotating fashion, as represented by thearrows defining rolls 3. Neighboring rolls 3 rotate in oppositedirections, and the liquid crystal directors (represented by themultiple dash lines in FIG. 1) tilt in the direction of rotation. Thestructure of the convective roll array depends upon the amplitude andfrequency of the electric field. Inside the cells, the nematic liquidcrystal undergoes circular flow and forms a highly regular pattern,accompanied by a distortion of the optical axis. Herein, this dynamicprocess is the basis for forming electro-convective diffractive devices,because it produces a periodic modulation of the liquid crystal opticalproperties.

In this invention, diffraction gratings are formed with nematic liquidcrystal samples confined between substrates, and these diffractiongratings operate at wavelengths determined by the repeat spacing of theconvective rolls into which the liquid crystals assemble. The spacingcan range up to about 1 mm, yielding infrared performance at wavelengthsup to about 250 μm. The grating spacing and structure factor can betuned during manufacture of the diffraction grating by controlling theamplitude and frequency of the applied electric field. The spatialperiodicity of the diffraction grating arises from the intrinsicproperties of the nematic liquid crystal materials.

More particularly, this invention relates to a diffraction device inwhich the diffractive element, i.e., the grating, is an array ofperiodic convective rolls that forms when a polymerizable mixture, whichcontains nematic liquid crystal and is disposed between substrates, issubjected to an electric field of appropriate frequency and amplitude.The polymerizable mixture includes nematic liquid crystal, dopant,polymerizable precursor, and a polymerization initiator. In aparticularly preferred embodiment, the polymerizable precursor isphotopolymerizable, and the initiator is a photoinitiator.

In general, the present invention provides a tunable diffraction gratingcomprising a cell with a first cell wall spaced from a second cell wall,with electrodes disposed on facing surfaces of the first and second cellwalls. An array of nematic liquid crystal convective rolls are arrangedperiodically in space between said first cell wall and said second cellwall, and a polymeric network stabilizes said array of nematic liquidcrystal convective rolls.

The present invention also provides a method for producing a diffractiongrating comprising the steps of providing a polymerizable mixture ofnematic liquid crystal, dopant, and polymerizable precursor, introducingthe polymerizable mixture between two electrically conductivesubstrates; applying a potential difference across the liquid crystal tocause the nematic liquid crystal to assemble into an array of convectiverolls; and stabilizing the convective roll structure by forming apolymer network from the polymerizable precursor, wherein the polymernetwork is templated by the convective roll structure.

DESCRIPTION OF THE DRAWINGS

For a complete understanding of the objects, techniques and structure ofthe invention, reference should be made to the following detaileddescription and accompanying drawings, and color photographs. The fileof this patent contains at least one photograph executed in color.Copies of this patent with the color photographs will be provided by thePatent and Trademark Office upon request and payment of the necessaryfee.

FIG. 1 is a schematic representation of electro-hydrodynamic convectionin nematic liquid crystal cells;

FIG. 2 is a representative cross-sectional view of a precursor to adiffraction grating according to this invention;

FIG. 3 is a color photomicrograph of a polymer stabilizedelectro-convective diffractive device; and

FIG. 4 is a color photograph of a far field diffraction pattern ofhelium-neon laser beam transmitted through a polymer stabilizedelectro-convective diffractive device.

PREFERRED EMBODIMENT FOR CARRYING OUT THE INVENTION

With reference to FIG. 2, a precursor to a diffraction grating accordingto this invention is designated generally by the numeral 10. Precursor10 includes a first cell wall 12 spaced from a second cell wall 14. Cellwalls 12, 14 can be any transparent material, for example, glass plates.Also, one of cell walls 12, 14 can be non-transparent. When one of cellwalls 12, 14 is non-transparent, the precursor 10 can be used in areflection mode.

An electrode 16 is disposed on an inner surface of cell wall 12 and anelectrode 18 is disposed on an inner surface of cell wall 14, such thatelectrodes 16, 18 are disposed on facing surfaces of the first andsecond cell walls 12, 14. The electrodes are any electrical conductingelectrodes. An example of a suitable electrode is an indium tin oxide(ITO) electrode. The electrodes are coated with a material such as apolymer, as is common in the art. Suitable polymers would includepolyimides, polyvinyl alcohols, and any other polymer that does notinterfere with the present invention. Polymer mixtures may also beemployed. The material on the electrode is aligned unidirectionally toprovide respective unidirectionally aligned alignment layers 20, 21disposed on each electrode 16, 18. Unidirectional alignment may beprovided through rubbing or photoalignment with polarized light or othermethods known or here after discovered. Alignment layer 20 or 21 or bothinduces a liquid crystal director, in the vicinity of wall 12 or 14, tolie parallel to the wall 12, 14, in a common direction. By way ofexample, and without limitation, alignment layer 20 may be an appliedrubbed polyimide alignment layer or an obliquely evaporated oxide or arubbed poly vinyl alcohol layer. Alternatively, an in situ polyimidelayer photopolymerized using polarized ultraviolet light may be used asthe alignment layer.

A polymerizable mixture 22 is disposed between first and second cellwalls 12,14. For reasons that will become more apparent hereinbelow,polymerizable mixture 22 includes a nematic liquid crystal, dopant, andpolymerizable precursor. This polymerizable mixture 22 is formed into anarray of nematic liquid crystal convective rolls 3 through applicationof an electric field across electrodes 16, 18, and the roll structure 3is stabilized through subsequent polymerization and/or cross-linking ofthe polymerizable precursor. As used herein, “polymerization and/orcross-linking” is to be understood as “polymerization or cross-linkingor both polymerization and cross-linking.”

A wide variety of liquid crystalline materials are potentially suitablefor polymerizable mixture 22. The liquid crystal material must possess anematic phase, and either positive conductivity anisotropy andsufficiently small dielectric anisotropy or negative conductivityanisotropy and sufficiently large dielectric anisotropy. For example,phenyl benzoates like p-methoxyphenyl-p′-hexyloxybenzoate,p-octyloxyphenyl-p′-pentyloxybenzoate or mixtures thereof. Othercandidates include Schiffs bases like methoxy benzylidene butylanaline,or alkoxy alkyl azoxybenzenes like 4-methoxy-4-butyl azoxybenzene ormixtures thereof. These materials are commercially available and may bereadily synthesized from commercially available precursors.

At least one dopant is present in the polymerizable mixture 22 in orderto induce the required electrical conductivity. That is, dopant ispresent so that an electric field applied across electrodes 16, 18 willcause the nematic liquid crystal material to self-assemble into aregular array of convective rolls 3 through the phenomenon known aselectro-hydrodynamic convection. This self-assembly causes a phaseseparation between the nematic liquid crystal and the polymerizableprecursors, and subsequent polymerization and/or cross-linking of thepolymerizable precursors will form a polymeric network that is templatedor bounded by the array of convective rolls. The polymeric networkstabilizes the convective roll structure. Useful dopants may be selectedfrom octyloxyphenol, pentyloxybenzoic acid, tetrabutyl ammonium bromide,and mixtures thereof. Charge transfer salts such as tetracyanoquinoneand related compounds may also serve as useful dopants.

The polymerizable precursors present in polymerizable mixture 22 maygenerally be selected from polymeric precursors capable of forming apolymer network in a controlled manner (i.e., when selectively activatedto form such a network). The precursor chosen must also be soluble inthe liquid crystal so that the desired phase separation and templatingmay occur upon the formation of the convective roll structure throughelectro-hydrodynamic convection. Particularly preferred polymerizableprecursors include, without limitation, bisphenol A dimethacrylate,other acrylates or methacrylates, vinyl ethers, styrenes or epoxies ormixtures thereof. The polymerizable precursor will be added at aconcentration of from about 1% to about 6% by weight of thepolymerizable mixture 22.

As mentioned, the polymerizable precursors are present for the creationof a stabilizing polymeric network templated by the self-assembled arrayof nematic liquid crystal convective rolls. The polymerizable precursorsare thus chosen so that they may be polymerized and/or cross-linked in acontrolled manner. Particularly, the polymerizable precursors should notpolymerize and/or cross-link (i.e., form a polymeric network) untilinitiated to do so, after formation of the convective rolls. Multiplemethods exist for controlling polymerization and/or cross-linking ofpolymeric precursors, and virtually any method may be employed thatensures that the polymeric precursors will not begin to form a polymericnetwork until the convective roll template is formed. Most commonly, inorder to control the formation of the polymeric network, an initiator ispart of polymerization mixture 22. The initiator may be triggered bylight (photoinitiator) or by heat (thermal initiation) or other means.As non-limiting examples of photoinitiators, they may be selected frombenzoketones such as benzoin methyl ether. The photoinitiator istypically added at a concentration of from about 0.1 to about 1% byweight of the polymerizable mixture 22. Polymerization and cross-linkingreactions and other methods for controlled formation of polymericnetworks that would stabilize the convective roll structure are known inthe art, and this invention is not limited to or by any particularmethod.

The polymerizable mixture 22 thus described is introduced via capillaryaction between first and second cell walls 12, 14, which are keptseparated from each other by appropriate spacers 24. The separation ofthe plates will generally range from about 5 microns to about 250microns.

After the cell is filled, wires are attached to electrodes 16, 18, andan electric potential difference of a chosen frequency and amplitude isapplied across the polymerizable mixture 22, as generally represented atthe numeral 26. Upon the application of the electric field, the liquidcrystal spontaneously self-assembles into an array of convective rolls(see FIG. 1). These rolls are arranged periodically in space, with agrating constant approximately twice the separation distance “d” betweenthe cell walls 12, 14 (e.g., glass plates), with variations around thisgrating constant resulting from the frequency of the applied electricfield. Thus, the grating constant is believed to be significantlydependent upon the distance between walls 12, 14 and the frequency ofthe applied electric field. The periodic roll structure is subsequentlystabilized by inducing the creation of a polymeric network from thepolymerizable precursors.

The application of the electric field (as at 26), and the subsequentcross-linking of the polymerizable precursor, provides a diffractiongrating from precursor 10. Polymerizable mixture 22 phase separates,with nematic liquid crystal formed into a periodic roll structure thatserves as a template for the creation of a polymeric network.

Polymerization and/or cross-linking is carried out, either throughphotoinitiation or thermal initiation or otherwise, while the electricpotential difference is applied to induce the convective roll structure.If common photoinitiators are present in polymerization mixture 22,ultraviolet light is directed onto the cell, and the photoinitiatorcauses polymerization and cross-linking of the polymerizable precursor.The duration of the illumination with ultraviolet light is typicallyshort, in the range of from about 2 to about 5 seconds. The optimalduration of exposure is believe to depend on the intensity of the lightas well as the concentration and particular formulation of precursor andphotoinitiator. If thermal initiators are used, the cell may be broughtto the necessary temperature for polymerization.

This procedure produces a stable diffraction grating that persists whenthe electric field is removed, and the resultant diffraction grating,more particularly, electro-convective diffractive device, has manyadvantages over diffraction gratings of the prior art. The presentdiffraction gratings are significantly quicker and easier tomanufacture, and may even be manufactured in vast quantities byemploying continuous roll-type processing methods. Furthermore, thediffraction grating can be manufactured using flexible substrates, suchas Mylar™ (DuPont, Delaware, USA), which enable the grating to beconformed to surfaces that are not flat. The ability to use flexiblesubstrates also provides diffraction gratings that are far more ruggedthan glass or replica gratings, which must be made out of rigid, brittlematerials.

The diffraction gratings of this invention are easily tunable, and thegrating constant may be varied over a wide spectrum by varying thefrequency and amplitude of the potential difference that is appliedbefore the formation of the polymeric network. When the distance betweenthe electrodes is fixed, the grating constant decreases as the frequencyincreases. Changes in amplitude also have some small effect on thegrating constant, although the distance between the walls (such as walls12, 14) and the frequency of the applied field are believed to be moredeterminative of the grating constant.

The structure factor, on the other hand, can be tuned afterpolymerization and/or cross-linking, during the operation of thediffraction grating. Although the convective roll structure, and, thus,the grating constant, is frozen in after the formation of the polymericnetwork, the structure of the liquid crystal within the rolls or theorientation of the liquid crystal directors within the roll can bealtered by application of an electric field. Thus, after the convectiveroll structure is stabilized by the templated formation of the polymericnetwork, the structure factor can be adjusted, but not the gratingconstant.

EXPERIMENTAL

A diffraction grating was produced according to the method providedabove. The liquid crystalline material was a mixture of phenylbenzoates, commonly known as Mischung V. Mischung V is composed of fourcompounds as follows:

Mischung V Components Weight % p-methoxyphenyl-p′-hexyloxybenzoate 22.0%p-octyloxyphenyl-p′-pentyloxybenzoate 30.3%p-heptyloxypenyl-p′-hexyloxybenzoate 13.3%p-hexylphenyl-p′-butyloxybenzoate 34.4%

1.74 weight % of equal parts octyloxyphenol and pentyloxybenzoic acidwas added as the dopant. 4.9 weight % of bisphenol A dimethacrylate wasadded as the polymerizable precursor, and 0.87 weight % of benzoinmethyl ether as the photoinitiator.

The resultant mixture was introduced via capillary action into a liquidcrystal sample cell that consisted of two thin, flat glass plates,placed parallel to one another and kept separated at a distance ofapproximately 18 microns. More particularly, the sample cells werepurchased from E.H.C. Co. of Tokyo, Japan. The separation of the platesis maintained, according to the manufacturer, by adding glass spheres ofparticular size to the adhesive used to fix the plates together. Eachplate has applied to it a layer of indium tin oxide to render thesurface of the glass electrically conductive. Each glass plate was alsorubbed with a polyimide alignment layer to induce the liquid crystaldirector to lie parallel to the plate.

Wires were attached to the conducting areas of each plate, an electricpotential difference was applied across the liquid crystal layer. Thefrequency was 13.7 Hz and the amplitude was 26.6V_(rms). The appliedpotential difference caused the spontaneous self-assembly of the liquidcrystal into an array of convective rolls. This array was thenstabilized by inducing cross-liking between the polymerizable precursormolecules by exposing the liquid crystal sample to ultraviolet light.

The liquid crystal sample was placed on a microscope stage. Themicroscope was equipped with an epi-illumination system and highpressure mercury lamp. More particularly, the lamp employed was an HBO103 W/2 short arch lamp manufactured by Osram of Berlin, Germany. Duringapplication of the electric potential difference, the sample cell wasilluminated with ultraviolet light for two to three seconds. Theluminous flux of the ultraviolet light was estimated at 0.6 cd/cm².

This experiment was successful in producing a stable diffraction gratingthat persisted when the electric field was removed. A micrograph of thisgrating is shown in FIG. 3, and a photo of the diffraction patternobtained by passing a helium neon laser through the grating at normalincidence is shown in FIG. 4.

Thus, it should be evident that the method of the present invention iseffective at producing diffraction gratings by spontaneous self-assemblythrough electro-hydrodynamic convection. Although an exemplaryembodiment of this method has been provided herein, the presentinvention should not be limited thereto or thereby. The claims willserve to define the invention.

1. A tunable diffraction grating comprising: a cell with a first cellwall spaced from a second cell wall; electrodes disposed on facingsurfaces of the first and second cell walls; and an array of nematicliquid crystal convective rolls, wherein said convective rolls arearranged periodically in a space between said first cell wall and saidsecond cell wall; and a polymeric network stabilizing said array ofnematic liquid crystal convective rolls, wherein a grating constant ofsaid tunable diffraction grating is determined by a structure of saidconvective rolls.
 2. The tunable diffraction grating of claim 1, whereinthe convective rolls are arranged with the grating spacing approximatelytwice the separation distance between said first and second cell walls.3. The tunable diffraction grating of claim 1, further comprising: apower source connected to said electrodes to apply an electric field,wherein said convective rolls are arranged with a structure factor, andsaid structure factor is adjusted by application of an electric fieldthough said power source.
 4. A method for producing a diffractiongrating comprising the steps of: introducing a polymerizable mixtureincluding nematic liquid crystal, dopant, and polymerizable precursorbetween two electrically conductive substrates; applying a potentialdifference across the polymerizable mixture to cause the nematic liquidcrystal to assemble into an array of convective rolls; and stabilizingthe convective roll structure by forming a polymer network from thepolymerizable precursor, wherein the polymer network is bounded by theconvective roll structure and a grating constant of said diffractiongrating is determined by the structure of said convective rolls.
 5. Themethod according to claim 4, wherein the polymerizable mixture furtherincludes an initiator, said initiator being activated in said step ofstabilizing to initiate the formation of the polymer network from thepolymerizable precursor.
 6. The method according to claim 5, wherein theinitiator is a photoinitiator and said step of stabilizing includesphotoinitiation of the photoinitiator.
 7. The method according to claim4, wherein said convective rolls are arranged with a structure factorafter said step of stabilizing, and the method further comprises, aftersaid step of stabilizing: adjusting the structure factor by applicationof an electric field though at least one of the electrically conductivesubstrates.